The present invention relates to a method of filling fibrous-structure electrode plaques for rechargeable batteries with an active compound paste, while at the same time sizing the electrode plaque.
Rechargeable batteries for storing electrical energy in the form of chemical energy (which can then be drawn off again as electrical energy) have been known for a long time. In the lead rechargeable batteries, which are still widely used, the electrodes or plates consist of the active material, which is the actual energy store, and a lead support (grid), which accommodates the active material. For some time there have also been rechargeable batteries containing a novel electrode type, in which an electrode plaque has a fibrous structure. Such batteries are well known to the prior art. For example, German Patent Specification 33 18 629 describes a metallized plastic fiber electrode plaque with a nonwoven fabric base for battery electrodes.
The activation and chemical metallization of nonwoven fabric and needle-punched felt sheets is disclosed in German Patent Specification 36 31 055 and German Patent Specification 36 37 130. German Patent Specification 38 17 828 and German Patent Specification 38 17 826 specify aqueous nickel hydroxide or cadmium oxide pastes for the vibration filling of foam-structure and fibrous-structure electrode plaques. A continuous filling method is furthermore to be found in German Patent Specification 38 22 197 and a method for the vibration filling of foam-structure of fibrous-structure electrode plaques in German Patent Specification 38 16 232. German Patent Specification 38 22 197 also incorporates the removal of the excess paste from the electrode plaque. German Patent Specification 36 32 352 specifies a fibrous-structure electrode plaque with welded-on current take-off terminal, while German Patent Specification 38 17 982 specifies the removal of the paste from the current take-off terminal after the impregnation process. Finally, German Patent Application P 40 18 486.2 specifies a method of producing fibrous-structure electrodes in which the plaque, sized before the mechanical impregnation, is re-sized by compression after the filling operation.
The above review, which is not represented as complete, shows that fibrous-structure electrode technology is at present a field which is being worked on intensively. In practice, however, it is repeatedly found that difficulties and shortcomings occur in the production of fibrous-structure electrode plaques. In particular, the method steps of sizing, filling and removing the excess paste have proven problematic.
Fibrous-structure electrodes are generally produced by a process in which, after activation, metallization and reinforcement by electroplating, the fibrous-structure sheet is cut to size and provided with a current take-off terminal. It is also sized before being filled with active compound, which is necessary in order to be able to produce electrode plaques with defined filling and small fabrication tolerance. In some cases, the fibrous-structure electrode plaque is even sized hot before filling in order to bind projecting fibers superficially, or it is flamed-off. In the latter case however, only the plastic fibers which are not nickel-plated ar reduced.
Most of the energy introduced in the sizing operation is plastic deformation energy, and the lesser part elastic deformation energy. If the active compound is introduced by vibration filling, 96% to 100% of the pores are filled with active compound in the form of known pastes. This process generates substantial noise (vibration of the electrodes, the vibration transmitter(s) or the paste pots), and substantial mess is produced in the form of spattered paste. Moreover vibration filling is difficult to automate.
A further disadvantage of the earlier procedure is that the fibrous-structure electrodes, after first being sized with effort, are stress-relieved by vibration during the impregnation and consequently undergo indeterminate increases in thickness. In addition, during withdrawal from the paste after filling, the electrodes retain enough mass of paste on their surface to correspond on average to approximately the paste mass in the interior of the electrode. This is true, in particular, for electrodes about 2.5 mm thick. For thicker electrodes, less mass than corresponds to the filling of active compound is withdrawn, but on the other hand, in the case of thinner electrodes the effect is the opposite, so that the paste carried out of the impregnation vessel is often a multiple of that which is actually introduced into the fibrous-structure electrodes. The paste adhering to the surface has to be removed by scrapers or brushes or by rotation in a centrifugal field in one or more method steps following the impregnation. In this process, ends of nickel strands not completely bound in the composite, which were pressed into the surface of the fibrous-structure electrode by sizing before impregnation, are additionally often torn out and project in some cases even at right angles from the electrode surface. This problem is particularly prevalent at the cut edges of the electrodes. After filling, cleaning the surface and drying the fibrous-structure electrode, its surface is anything other than flat. As an additional layer, a dried-on film or striation of the paste due to surface cleaning also has a subsequent adverse effect when the fibrous-structure electrode is used.
When such fibrous-structure electrodes are used in the production of cells having prismatic shape, the cell housing becomes quite thick after the assembly of the plate stack and the installation of the plate stack with separators and recombiners as a result of unduly large variations in the individual manufacturing steps, inter alia in the thickness of the individual components (chiefly the positive and negative electrodes). As a result a plurality of such cells cannot be installed in an available steel container (battery tray).
The unduly large manufacturing tolerances in the production of the electrode can also give rise to further disadvantages in the construction and operation of cells containing parts produced in this manner; in particular: that the designed amount of electrolyte corresponding to the volume of the planned housing does not match the volume of the actual, enlarged housing, that the theoretical and calculated porosities and cavity distributions are not achieved in the real cell, that shifts occur in the level of the dischargeable capacity and energy for various loads, that lower Ah and Wh outputs are achieved, that a modified internal cell pressure is established (usually associated with a reduced life of the cell), that a uniform electrode spacing is not ensured as a result of the undefined electrode geometry, that a nonuniform distribution of the amount and concentration of the electrolyte occurs, that the pressure conditions at the individual separators and, consequently, a uniform electrolyte storage (take-up capacity) are disturbed, or that an imbalance results in the fraction of the charge and discharge margin of the negative electrode, in which the negative electrode is greater than the positive electrode.
The resultant roughness, due, inter alia, to a plaque swelling during vibration filling, inadequate removal of all the excess paste from the surface of the electrode after pasting and failure to eliminate such resultant roughness before assembly, result in a high failure rate of the cells due to short circuits. Although resizing after impregnation and drying of the fibrous-structure electrodes does eliminate some of these shortcomings, it is a further operational step in which additional environmental protection measures are required due to the possible dust formation by the dried active material.
The object of the present invention is therefore to provide a method of filling fibrous-structure electrode plaques having current take-off terminals for rechargeable batteries, with an active compound paste under the action of compressive and frictional forces, with simultaneous sizing of the fibrous-structure electrode plaque without the disadvantages specified. In such a procedure, the fibrous-structure electrode plaques produced should have only a small tolerance in the filling, and the individual operational steps previously necessary in filling the fibrous-structure electrode plaque should be carried out in a single method step. That is, the sizing previously carried out before filling the plaque, vibration filling of the plaque, removal of the excess paste from the surface of the plaque after the filling operation and establishment of the final dimensional accuracy necessary for use by a further sizing of the plaque filled with the active compound paste, should therefore be carried out in one working operation.
This object is achieved, according to the invention in which a fibrous-structure electrode plaque with welded-on current take-off terminal is fed vertically from above to two rollers which are situated opposite each other horizontally, and which move in opposite directions to define a feed side in which materials inserted therein will be pulled through the two rollers. The roller nip is adjusted to the thickness of the fibrous-structure electrode to be manufactured with the welded-on current take-off terminal. In setting the nip width, it must be taken into account that the sized fibrous-structure electrode enlarges again after the sizing operation, by the corresponding amount of elastic deformation energy introduced in sizing. The amount of enlargement depends, inter alia, on the nickel coating of the plaque, the ratio of the thickness of the sized electrode to the initial electrode, the modulus of elasticity of the plaque, the cross-linking factor of the nickel-plated fibers in the plaque, the roll diameter of the roll mill and the throughput speed. The working width of the roll mill is set by right-hand and left-hand material strippers at the two roller ends to a working width which corresponds to the width plus the upper tolerance of the electrode to be manufactured.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.